CP/MAS 13C NMR study of pulp hornification using nanocrystalline cellulose as a model system

► Hornification of paper pulp is studied using solid-state NMR. ► Nanocrystalline cellulose (NCC) is used as model for the crystalline parts of pulp. ► Signs of hornification can be seen for both paper pulp and NCC after drying. ► Conclusion is that the crystalline parts of paper pulp contribute to hornification. ► Elements of higher hydrophobicity in NCC experiences increased aggregation. The hornification process of paper pulp was investigated using solid-state 13C NMR spectroscopy. Nanocrystalline cellulose was used to serve as a model system of the crystalline parts of the fibrils in pulp fibers. Characterization of the nanocrystalline cellulose dimensions was carried out using scanning electron microscopy. The samples were treated by drying and wetting cycles prior to NMR analysis where the hornification phenomenon was recorded by spectral changes of the cellulose C-4 carbon signals. An increase of the crystalline signal and a decrease of the signals corresponding to the accessible amorphous domains were found for both paper pulp and nanocrystalline cellulose. These spectral changes grew stronger with repeating drying and wetting cycles. The results show that cellulose co-crystallization contribute to hornification. Another conclusion is that the surfaces of higher hydrophobicity in cellulose fibrils have an increased preference for aggregation.